Arterial oxygen measurements (obtained invasively) are frequently used for measuring the physiological or "effective" shunt fraction resulting from gas exchange units having lower than average ventilation/perfusion ratios. Recently, a method employing inert gas insertion and blood concentration measurement (both invasive) of the inert gases has demonstrated the capability of measuring the distribution of low V/Q units and accurately predicting the resulting reduced arterial oxygen concentration. The purpose of this proposal is to demonstrate the feasability of developing a noninvasive means of both inert gas insertion and arterial blood concentration measurement for the purpose of monitoring the shunt fraction distribution and the arterial oxygen concentration noninvasively. A transcutanious method, analogous to the transcutaneous method for obtaining aterial oxygen estimates is to be employed. A major difference is that inert gases are undisturbed by tissue metabolism while skin metabolism, coupled with an uncertain skin perfusion, can defeat the intent of the measurement. A major advantage of the inert gas transcutaneous approach is that it should not be defeated by physiological uncertainties; however, the major problem with the approach is that sensitivity requirements for a skin sensing unit exceed the minimum detection limits for the most sensitive gas sensors available. The major objectives of the proposal are 1) to experimentally verify the sensitivity requirements for the method, and 2) construct a working model of a sensing device which meets these sensitivity requirements. The concept for the gas sensing device was generated from recognition of the following favorable circumstances: 1) electron capture, the most sensitive detection principle for some specific substances, is concentration dependent which allows miniaturization without compromising sensitivity, 2) the gases most easily detected by electron capture are the most suitable for the method, and 3) these inert gases can be concentrated by Peltier aided molecular sieve trapping which lends itself to miniaturization. A technically favorable feature of the method is that its accuracy depends primarily on measurements of ratios and the ratios of membrane solubilities of the various gases.